Iron-Histidine Coordination in Cytochrome b5: A Local Vibrational Mode Study

For a series of cytochrome b5 proteins isolated from various species, including bacteria, animals, and humans, we analyzed the intrinsic strength of their distal/proximal FeN bonds and the intrinsic stiffness of their axial NFeN bond angles. To assess intrinsic bond strength and bond angle stiffness, we employed local vibrational stretching force constants k^a(FeN) and bending force constants k^a(NFeN) derived from the local mode theory developed by our group; the ferric and ferrous oxidation states of the heme Fe were considered. All calculations were conducted with the QM/MM methodology. We found that the reduction of the heme Fe from the ferric to the ferrous state makes the FeN axial bonds weaker, longer, less covalent, and less polar. Additionally, the axial NFeN bond angle becomes stiffer and less flexible. Local mode force constants turned out to be far more sensitive to the protein environment than geometries; evaluating force constant trends across diverse protein groups and monitoring changes in the axial heme-framework revealed redox-induced changes to the primary coordination sphere of the protein. These results indicate that local mode force constants can serve as useful feature data for training machine learning models that predict cytochrome b5 redox potentials, which currently rely more on geometric data and qualitative descriptors of the protein environment. The insights gained through our investigation also offer valuable guidance for strategically fine-tuning artificial cytochrome b5 proteins and designing new, versatile variants.